Coral reefs suffer 4th global bleaching event due to climate change

Aerial photo taken on June 2, 2021 shows the Great Barrier Reef in Queensland, Australia. (Photo by Hu Jingchen/Xinhua)

As the world's oceans continue to warm, coral bleaching is becoming more frequent and severe.

BEIJING, May 22 (Xinhua) -- The world is currently experiencing its fourth global coral bleaching event, the second in a decade, which is caused by high ocean temperatures brought on by climate change, according to a recent report.

Mass bleaching of coral reefs since early 2023 has been confirmed in at least 53 countries, territories and local economies, including Australia's Great Barrier Reef, the largest coral reef in the world, according to an announcement jointly released by the U.S. National Oceanic and Atmospheric Administration and the International Coral Reef Initiative on April 15. Global coral bleaching events last occurred in 1998, 2010, and from 2014 to 2017.

According to a report published by the federal government-funded Great Barrier Reef Marine Park Authority in April, the UNESCO World Heritage-listed reef is experiencing its worst mass bleaching event on record. Almost three-quarters of the reef shows signs of bleaching, and nearly 40 percent shows high or extreme bleaching.

"From Feb. 2023 to April 2024, significant coral bleaching has been documented in both the Northern and Southern Hemispheres of each major ocean basin," said Derek Manzello, coordinator for the National Oceanic and Atmospheric Administration's Coral Reef Watch program.

Coral bleaching occurs when corals experiencing heat stress expel the symbiotic algae living in their tissue, turning them white. Bleached coral is not dead but is more likely to die from starvation due to a disruption in its photosynthesis process.

Researchers have found that the causes of coral bleaching include abnormal sea temperatures, ultraviolet radiation, storms, pollution, and bacterial or viral infections. However, the primary cause of large-scale coral bleaching is prolonged periods of abnormally high ocean temperatures.

"When seawater temperatures are higher than normal for a longer period, corals begin to suffer from heat stress. The high temperatures disrupt the symbiosis between the corals and the microalgae (zooxanthellae) living in their tissue. The algae then produce molecules that damage both the algae themselves and the coral," said Mathilde Godefroid, who researches coral reefs at the Max Planck Institute for Marine Microbiology in Germany.

"In response, the corals expel their symbiotic partners to minimize the damage," she said, adding that although this helps the coral cells for a short time, it harms them in the long term, as the microalgae cover up to 90 percent of the coral's energy requirements through photosynthesis.

The algae also give the corals their color. When they are shed, the coral tissue becomes transparent, and the white skeleton underneath is revealed: the corals, therefore, appear bleached, she said.

"If the ambient temperatures return to normal relatively quickly, the corals can rebuild their symbionts and survive bleaching, even if this naturally puts them under a lot of stress. However, if the increased temperatures persist or other stress factors are added, the corals can no longer recover. Then they die," she explained.

"As the world's oceans continue to warm, coral bleaching is becoming more frequent and severe," Manzello said. "When these events are sufficiently severe or prolonged, they can cause coral mortality, which hurts the people who depend on the coral reefs for their livelihoods."

The Great Barrier Reef has seen five mass bleaching events in the past eight years. Climate-change-driven increases in ocean temperatures are making it more difficult for the reef's corals to recover between such events, said marine biologist Terry Hughes at James Cook University in Townsville, Australia, in a recent report on the website of the journal Nature.

Coral reefs play a crucial role in marine ecosystems and are closely linked to developing industries such as fisheries and tourism.

The extent and speed of global warming are decisive for the future of the world's coral reefs: with a temperature increase of 1.5 degrees, 70 to 90 percent of the reefs are expected to disappear; with an increase of two degrees, possibly by 2050, almost all the world's reefs will die off.

However, the pace of warming is also significant: a slower temperature rise allows corals to adapt. "So if we manage to limit warming and take local protective measures, we may be able to preserve some reefs or some resilient coral species," said Godefroid.